The invention relates generally to a method of and an apparatus for the continuous casting of metals, e.g., steel.
More particularly, the invention relates to a method of and an apparatus for conveying a continuously cast strand.
In the continuous casting of steel, the strand formed in the continuous casting mold is continuously withdrawn from the latter and then advanced through a secondary cooling zone. The secondary cooling zone may include a guide for the strand, and such guide is normally made up of pairs of guide rollers arranged in a row.
The strand is drawn out of the mold and through the guide by one or more drive units which, as a rule, comprise cooperating pinch rollers. However, drive units consisting of driven guide rods designated "walking beams" are also known. In addition to strand withdrawal, the drive units serve to feed a dummy bar into the mold before the start of a casting operation.
During a continuous casting operation, the strand is drawn out of the mold and the strand guide at as uniform a speed as possible. Although it is desirable to keep the strand moving continuously at a uniform speed throughout the casting operation, there are nevertheless situations in which the supply of steel to the mold and withdrawal of the strand must be interrupted. This can occur when there is a disturbance in operation or when a tundish change is performed during a long casting cycle. In such situations, and also at the end of the casting operation, the bath in the mold is slagged off while the strand is kept stationary or is moved at a creep. The strand is solidified by cooling with water and is subsequently drawn out of the mold at low speed.
Both the strand and the continuous casting apparatus are adversely affected whenever the strand is stopped or moved at a creep. The ferrostatic pressure in the strand causes bulging of the latter to an extent depending upon the previous casting speed, the size and/or shape of the strand, and the pitch of the rollers of the strand guide. When the drive units are restarted to withdraw the strand from the apparatus, the bulges are rolled flat thereby leading to internal cracking of the strand.
If the strand is stationary, the size of the bulges can be reduced by intense cooling. However, this leads to supercooling of localized areas of the strand while adjacent areas are not cooled at all or are cooled only slightly. Such variations in strand cooling generate surface cracks in the strand. Moreover, in continuous casting apparatus having a curved strand guide, the stress on the straightening rollers and their bearings is increased several times when supercooled sections of a strand are transported through the straightening unit.
As already mentioned, strand stoppages are also detrimental to the continuous casting apparatus, especially the rollers. The intense and, depending upon the stoppage time, prolonged heating of a roller surface in contact with a hot strand leads to surface cracks in the roller. Furthermore, the cooling of the portion of the roller facing away from the strand causes deformation and bending of the roller. The result is permanent roller bending and non-circular running of the roller leading to overloading of the bearings and the strand guide. Permanent roller bending, like the removal of bulges by rolling, is further detrimental to the strand. In addition, bent rollers cause the strand to be improperly guided which, in turn, leads to overloading of the rollers and their drive motors. This may necessitate interruption of the casting operation.
In order to avoid or reduce faulty operation and damage to the strand and rollers, it has been proposed to construct the rollers with multiple supports and large drive motors, and even to provide heavy additional withdrawal stands. Expensive roller designs and roller coatings, which make the continuous casting apparatus more expensive and increase maintenance costs, have also been proposed in order to render the rollers insensitive to surface cracks. However, all such measures are undesirable because of the relatively great expense associated therewith, particularly in the case of slab casters having a large width and bloom casters having long strand guides.